In combustion chambers (called combustors) of turbine engines, acoustic vibrations can occur during the combustion process under certain conditions due to instabilities in the combustion process. In the industry, these high frequency acoustic vibrations are sometimes referred to as oscillations. Oscillations have been found to interfere with optimal operation of the turbine engine. Once oscillations occur, they can continue until the source of energy causing the oscillations is removed, or until system variables are changed, to shift the operation of the turbine engine to a non-oscillations operational range. However, the mechanics of how the operational characteristics interact to produce oscillations is not well understood. Therefore, changing the operational characteristics of the turbine engine to eliminate oscillations may be difficult since it is difficult to predict oscillations in a system with sufficient accuracy. Therefore, a positive structural means, such as a Helmholtz resonator, may be designed into the combustor to damp the high frequency acoustic vibrations.
A Helmholtz resonator, in its simplest form, consists of an enclosed volume (cavity) containing air connected to the combustion chamber with an opening. Due to a pressure wave resulting from the combustion process, air is forced into the cavity increasing the pressure within the cavity. Once the external driver that forced the air into the cavity is gone, the higher pressure in the cavity will push a small volume of air (plug of air) near the opening back into the combustion chamber to equalize the pressure. However, the inertia of the moving plug of air will force the plug into the combustion chamber by a small additional distance (beyond that needed to equalize the pressure), thereby rarifying the air inside the cavity. The low pressure within the cavity will now suck the plug of air back into the cavity, thereby increasing the pressure within the cavity again. Thus, the plug of air vibrates like a mass on a spring due to the springiness of the air inside the cavity. The magnitude of this vibrating plug of air progressively decreases due to damping and frictional losses. The energy of the pressure wave generated within the combustor is thus dissipated by resonance within the Helmholtz resonator. Energy dissipation is optimized by matching the resonance frequency of the Helmholtz resonator to the acoustic mode of the combustor. Typically, frequency matching (or “tuning”) of a Helmholtz resonator is accomplished by changing the dimensions of the Helmholtz cavity and the opening.
An array of Helmholtz resonators can be constructed using an empty space between interior and exterior liners of a double walled combustor. However, in such double walled combustors, the space between the liners is used to supply cooling air to the combustor walls. Therefore, locating the Helmholtz resonators in this space makes them a part of the cooling system. Helmholtz resonators being a part of the cooling system, reduces the ability to tune the Helmholtz resonators by changing the cavity and opening dimensions, without impacting the cooling of the combustor. This limitation reduces the effectiveness of the Helmholtz resonators in controlling oscillations. It is therefore desirable to locate the Helmholtz resonators close to the heat release zone of the combustor, but independent of the combustion chamber cooling system.
One implementation of a Helmholtz resonator in a gas turbine combustion chamber is described in U.S. Pat. No. 7,104,065 (the '065 patent) issued to Benz et al. on Sep. 12, 2006. In the '065 patent, Helmholtz resonators are located outside the outer liner of a double walled combustor. A throat section that penetrates through the inner and outer liner fluidly couples the resonator cavity with the combustor volume within the inner liner. In the '065 patent, a welded joint is used between the throat section of the resonator and the wall of the combustor to ensure a gas tight seal. By locating the Helmholtz resonator outside the space between the inner and outer liner, the '065 patent separates the resonator cavity from the cooling air path between the inner and outer liner.
Although the Helmholtz resonator of the '065 patent may be tuned without affecting the gap between the inner and the outer liner, the combustor of the '065 patent may have other drawbacks. For instance, the Helmholtz resonators on the outer liner may affect the cooling air flow into the space between the inner and the outer liner. Furthermore, thermo-mechanical stresses may develop at the welded joints between the throat and the liner due to thermal expansion mismatch between these parts. These thermo-mechanical stresses may eventually lead to cracks in the welded joints (or the attached parts) that compromise the reliability of the combustor.
The present disclosure is directed at overcoming one or more of the shortcomings set forth above.